Heretofore, in the semiconductor industry, photolithography methods using visible light or UV light have been employed as a technique of transferring fine patterns required for forming an integrated circuit constituted by fine patterns, on e.g. a Si substrate. However, the miniaturization of semiconductor devices is rapidly achieved and is brought close to its limitation obtainable by conventional light exposure methods. In the case of light exposure, the resolution limit of a pattern is about a half of the wavelength used for exposure, and it becomes about a quarter of the wavelength used for exposure even if an immersion method is used. The resolution limit is expected to be about 45 nm even by an immersion method using an ArF laser (193 nm). To cope with this problem, as the exposure technique for 45 nm or smaller patterns, EUV lithography, which is an exposure technique using EUV light having a shorter wavelength than that of an ArF laser, is considered to be prospective. In this specification, EUV light means a light beam having a wavelength in a soft X-ray region or vacuum UV region, and specifically, it means a light beam having a wavelength of about from 10 to 20 nm, particularly about 13.5 nm±0.3 nm.
Since EUV light tends to be absorbed by all materials and the refractive indexes of all materials for EUV light are close to 1, it is not possible to use a refractive optical system such as ones used for the conventional lithography methods using visible light or UV light. For this reason, in EUV lithography, a reflective optical system, that is, a reflective photomask and mirrors, are employed.
A mask blank is an unpatterned laminate for producing a photomask. A mask blank for a reflective photomask has a structure that a reflective layer reflecting EUV light and an absorptive layer absorbing EUV light are formed on a substrate of e.g. glass in this order. As the reflective layer, a multilayer reflective film is usually employed, which comprises high refractive index layers and low refractive index layers that are alternately laminated, and which has high light beam reflectivity when a light beam is incident into a surface of the layer, more specifically, high light beam reflectivity when EUV light is incident into a surface of the layer. For the absorptive layer, a material having high absorption coefficient for EUV light, specifically, for example, a material containing Cr or Ta as a main component, is employed.
The multilayer reflective film and the absorptive layer are formed by using an ion beam sputtering method or a magnetron sputtering method. At the times of forming the multilayer reflective film and the absorptive layer, the substrate is supported by a supporting means. Examples for the means of supporting a substrate include a mechanical chuck and an electrostatic chuck. However, from the viewpoint of particle generation, an electrostatic chuck is preferably employed. Further, in a mask patterning process or mask handling for exposure, an electrostatic chuck is employed as the means for supporting a substrate. However, in a case of a substrate such as a glass substrate having a low dielectric constant and a low conductivity, it is necessary to apply a high voltage to obtain a clamping force equivalent to that required for clamping a silicon wafer, and there is a risk of causing a dielectric breakdown. In order to solve this problem, Patent Document 1 describes a mask substrate having a rear surface coating (conductive film) formed of a material, such as Si, Mo, chromium oxynitride (CrON) or TaSi other than commonly used Cr, which has a higher dielectric constant and a higher conductivity than those of glass substrate, as a layer for promoting the electrostatic chucking of the substrate.
However, in the mask substrate described in Patent Document 1, since the CrON film has a low adhesion to the mask substrate, there is a problem that peeling occurs between the glass substrate and the CrON film at the time of forming a multilayer reflective film or an absorptive film with the result that particles are formed. Particularly, in the vicinity of the interface between the electrostatic chuck and the CrON film, peeling of the film tends to be caused by a force applied to the vicinity of the interface between the substrate and the electrostatic chuck, which is produced by rotation of the substrate.
Further, in the mask substrate described in Patent Document 1, a conductive film is formed on the entire region of one surface as well as chamfers and side faces of the substrate. Accordingly, the adhesive forces of the film to the chamfers and side faces of the substrate are particularly weak since the conductive film is obliquely formed on the chamfers and side faces, and the peeling of the film tends to be caused by warpage of the substrate at a time of clamping by an electrostatic chuck or by contact with an end effecter of a robot arm.
Further, in the mask substrate described in Patent Document 1, since oxygen (O) and carbon (C) are contained in large amounts in a surface of the CrON conductive film, abnormal discharge may occur in the process of forming a multilayer reflective film or absorptive film in some film-forming conditions.
Such peeling of a conductive film at a time of e.g. clamping by an electrostatic chuck (at a time of film-forming) or particle generation due to abnormal discharge at a time of film-forming, increases defects in a product (a substrate with a multilayer reflective film, a reflective mask blank for exposure or a reflective mask for exposure), and prevents production of high quality product. In a case of pattern transfer by using a conventional transmission mask for exposure, since the wavelength of exposure light is relatively long in a UV region (about 157 to 248 nm), even if a concave or convex defect is formed on a mask surface, a critical problem is unlikely caused, and accordingly, the generation of particles at a time of film-forming has not been recognized as a major problem. However, in a case of using light having a short wavelength such as EUV light as exposure light, even a fine concave or convex defect on a mask surface has a major influence on pattern transfer, and accordingly, the generation of particles cannot be ignored.
In order to solve the above problems, Patent Document 2 discloses a substrate with a multilayer reflective film in which particle generation due to peeling of a conductive film at a time of clamping by an electrostatic chuck or generation of particles due to abnormal discharge are prevented; a high quality reflective mask blank for exposure having few surface defects due to particles; and a high quality reflective mask for exposure having no pattern defect due to particles.
In order to solve the above problems, the substrate with a multilayer reflective film described in Patent Document 2 comprises a conductive film wherein the material forming the conductive film changes in the thickness direction of the conductive film so that a substrate side of the conductive film contains nitrogen (N) and a surface side of the film contains at least one of oxygen (O) and carbon (C). With respect to the reason why the conductive film has such a structure, Patent Document 2 describes that nitrogen (N) contained in the substrate side of the conductive film improves the adhesion of the conductive film to the substrate to prevent the conductive film from peeling, and that nitrogen reduces the film stress of the conductive film to allow increase of the attractive force between an electrostatic chuck and the substrate. Further, at least one of oxygen (O) and carbon (C) contained in the surface side of the conductive film increases the surface roughness of the conductive film to an appropriate level, to increase attractive force between an electrostatic chuck and the substrate at a time of clamping by the electrostatic chuck, to thereby prevent abrasion between the electrostatic chuck and the substrate. Here, oxygen (O) contained in the conductive film roughens the surface roughness (increases the surface roughness) to an appropriate level, and increases the attractive force between the electrostatic chuck and the substrate, and carbon (C) contained in the conductive film decreases the specific resistance of the conductive film to thereby improve the attractive force between the electrostatic chuck and the substrate, according to this document.
Here, in the substrate with a multilayer reflective film described in Patent Document 2, a metal material contained in the conductive film is at least one type selected from the group consisting of chromium (Cr), tantalum (Ta), molybdenum (Mo) and silicon (Si), and among these, chromium (Cr) is preferred. When the conductive film is made of a material containing chromium (Cr), the content of nitrogen (N) in the substrate side of the conductive film is preferably from 1 to 60 atomic % according to the document. Further, in a case of CrN, the content of nitrogen (N) is preferably from 40 to 60 atomic % according to the document. Meanwhile, the content of oxygen (O) in the surface side of the conductive film is preferably from 0.1 to 50 atomic %, and the content of carbon (C) is preferably from 0.1 to 10 atomic % according to this document.    Patent Document 1: JP-A-2003-501823    Patent Document 2: JP-A-2005-210093